5.1 It is possible for insulating materials used in high-voltage equipment to be subjected to transient voltage stresses, resulting from such causes as nearby lightning strokes. This is particularly true of apparatus such as transformers and switchgear used in electrical-power transmission and distribution systems. The ability of insulating materials to withstand these transient voltages is important in establishing the reliability of apparatus insulated with these materials.

5.2 Transient voltages caused by lightning will be of either positive or negative polarity. In a symmetrical field between identical electrodes, the polarity has no effect on the breakdown strength. However, with dissimilar electrodes there can be a pronounced polarity effect. It is common practice when using dissimilar electrodes, to make negative that electrode at which the higher gradient will appear. When asymmetrical electrodes are used for testing materials with which the tester has no previous experience or knowledge, it is recommended that he make comparative tests with positive polarity and negative polarity applied to the higher gradient, or smaller electrode, to determine which polarity produces the lower breakdown voltage.

5.3 The standard wave shape is a 1.2 by 50-μs wave, reaching peak voltage in approximately 1.2 μs and decaying to 50 % of peak voltage in approximately 50 μs after the beginning of the wave. This wave is intended to simulate a lightning stroke that strikes a system without causing failure on the system.

5.4 For most materials, the impulse dielectric strength will be higher than either its power frequency alternating voltage or its direct voltage dielectric strengths. Because of the short time involved, dielectric heating and other thermal effects are largely eliminated during impulse testing. Thus, the impulse test gives values closer to the intrinsic breakdown strength than do longer time tests. From comparisons of the impulse dielectric strength with the values obtained from longer time tests, it is possible to draw inferences as to the modes of failures under the various tests for a given material. Refer to Appendix X1 of Test Method D149 for further information on this subject.

1.1 This test method covers the determination of dielectric strength of solid electrical insulating materials under simulated-lightning impulse conditions.

1.2 Procedures are given for tests using standard 1.2 by 50 μs full-wave impulses.

1.3 This test method is intended for use in determining the impulse dielectric strength of insulating materials, either using simple electrodes or functional models. It is not intended for use in impulse testing of apparatus.

1.4 This test method is similar to IEC Publication 243-3. All procedures in this test method are included in IEC 243-3. Differences between this test method and IEC 243-3 are largely editorial.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Specific precaution statements are given in Section 9.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.